1. Field of the Invention
The present invention relates to a surface-mount type crystal unit, and in particular, to a surface-mount type crystal unit preferably used for a temperature compensated crystal oscillator and the like.
2. Description of the Related Art
Since a surface-mount type quartz crystal unit in which a quartz crystal blank is hermetically sealed in a surface-mounting package is small and lightweight, for example, it is incorporated in a crystal oscillator as a reference source of a frequency or time in portable electronic equipment. By detecting operating temperature of a crystal unit and compensating a frequency change on the basis of a frequency-temperature characteristic of the crystal unit, a temperature compensated crystal oscillator among such crystal oscillators supplies an oscillation signal at a fixed frequency regardless of a change of the ambient temperature. FIG. 1 illustrates construction of an example of a conventional temperature compensated crystal oscillator, and FIG. 2 illustrates a typical crystal blank used in a crystal unit. The temperature compensated crystal oscillator illustrated in FIG. 1 is a surface-mount type one which can be surface-mounted on a wiring board.
The shown temperature compensated crystal oscillator is constructed by bonding surface-mount type crystal unit 1 to mount substrate 2. Crystal unit 1 accommodates crystal blank 4 in package body 3 which is made of laminated ceramic, and in which a concavity is formed in one principal surface. In package body 3, frame wall layer 3b which has a rectangular opening in a center part is stacked on approximately rectangular bottom wall layer 3a, and the concavity for accommodating crystal blank 4 is formed of the opening of frame wall layer 3b. Hence, an inner bottom surface of the concavity of package body 3 is a portion where a top face of bottom wall layer 3a is exposed. In the inner bottom face of such concavity 10, one pair of crystal holding terminals 5 is provided in positions of both ends of one side of the inner bottom face with approaching each other. One pair of external terminals 6 is formed on an outer bottom surface of package body 3, that is, another principal surface of package body 3. Crystal holding terminals 5 are connected to external terminals 6 electrically through conduction paths formed on a stacking surface of bottom wall layer 3a and frame wall layer 3b in package body 3. External terminal 6 electrically connected to crystal holding terminals 5 is provided, for example, in one diagonal part of the outer bottom surface of package body 3, and external terminal 6 for grounding is provided in another diagonal part.
As shown in FIG. 2, for example, crystal blank 4 is an AT-cut quartz crystal blank having an approximately rectangular shape, and excitation electrodes 7 are formed in both of principal surfaces thereof, respectively. Each extending electrode 8 is extended toward both sides of the one end part of crystal blank 4 from the pair of excitation electrodes 7. In a position of the end part of crystal blank 4, extending electrodes 8 are formed so as to fold back between both principal surfaces of crystal blank 4. Crystal blank 4 is fixedly held in the concavity of package body 3 by fixing these extending electrodes 8 to crystal holding terminals 5 with, for example, electrical conductive adhesive 9 or the like in the positions, where the pair of extending electrodes 8 are led, respectively, and are then connected to external terminals 6 electrically.
A metal ring (not shown) which surrounds the concavity in package body 3 is provided on the top face, that is, the open end surface, of frame wall layer 3b of package body 3, and metal cover 10 is bonded to the metal ring by seam welding. Thereby, crystal blank 4 is hermetically sealed in the concavity.
Similarly to package body 3, mount substrate 2 is also made of laminated ceramics formed in a concavity. In an inner bottom surface of the concavity of mount substrate 2, IC (integrated circuit) chip 11 is fixed, for example, by flip-chip bonding, and thereby, IC chip 11 is accommodated in the concavity. At least an oscillation circuit which uses crystal blank 4, and a temperature compensation mechanism which compensates a frequency-temperature characteristic of crystal blank 4 are integrated in IC chip 11.
Bonding terminals are formed on a surface of square corners of an open end surface of the concavity of mount substrate 2. These bonding terminals are bonded by soldering or the like to external terminals 6 on the outer bottom surface of package body 3 of crystal unit 1, and thereby, mount substrate 2 and crystal unit 1 are unified. Bonding terminals are electrically connected to IC chip 11 through the conduction paths provided in the mount substrate.
In such a temperature compensated crystal oscillator, ambient temperature is detected by a temperature sensor provided in the temperature compensation mechanism in IC chip 11, the temperature compensation mechanism generates a compensation voltage according to the detected ambient temperature, and the generated compensation voltage is applied to a voltage variable capacitance element provided in an oscillation closed loop of the oscillation circuit. Thereby, since load capacity in a circuit side which is seen from crystal unit 1 as an inductor component changes, a change of an oscillating frequency by temperature is compensated for the oscillating frequency to be kept at a reference frequency (nominal frequency).
However, in the temperature compensated crystal oscillator with the above construction, since IC chip 11 which has the temperature sensor is accommodated in mount substrate 2 and crystal blank 4 which constructs crystal unit 1 is accommodated in package body 2, both are arranged in different spaces respectively. In addition, since generating heat by active elements, such as an oscillation amplifier which forms the oscillation circuit, IC chip 11 becomes at temperature higher than environmental temperature of crystal blank 4. Hence, since detection temperature by the temperature sensor in IC chip 11 becomes different from actual operating temperature of crystal blank 4, a problem that the frequency-temperature characteristic of the crystal unit cannot be sufficiently compensated arises.
Japanese Patent Laid-Open No. 2006-304110 (JP-A-2006-304110) discloses a temperature compensation crystal oscillation module in which a crystal unit and a thermistor for temperature detection are arranged on one principal surface of a ceramic substrate, and an IC chip is arranged on the other principal surface. In this crystal oscillation module, although an influence of heat generation of the IC chip is reduced since the thermistor for temperature detection is separately arranged from the IC chip, the thermistor is arranged in the external of the crystal unit, and hence, strictly speaking, it is not possible to detect the actual operating temperature of the crystal unit exactly, and it is not possible to compensate further adequately the frequency-temperature characteristic accordingly.